EXPLORATION OF TITAN
T
The Exploration of Titan
Ralph D. Lorenz
he exploration of Saturn’s moon Titan is reviewed, noting the dramatic recent
improvements in our knowledge of this strange world from the ongoing international Cassini-Huygens mission, results from which are summarized.
Among Cassini’s discoveries are the remarkable richness and complexity of its organic
chemistry, a strikingly Earth-like landscape with hydrocarbon lakes and seas, and vast
fields of organic sand dunes interspersed with craters and icy mountains. The breadth
of scientific questions posed by Titan’s Earth-like landscape and meteorology, and its
organic-rich prebiotic chemistry, set the stage for Titan to become a key target for
future solar system exploration, possibly by aircraft exploiting its thick atmosphere and
low gravity.
INTRODUCTION
The most Earth-like body in the solar system is not
another planet but Saturn’s largest moon, Titan.1 Indeed,
if Titan orbited the Sun rather than Saturn, we would
not hesitate to call it a planet in its own right. This
strange new world is larger than the planet Mercury
and has a thick nitrogen atmosphere laden with organic
smog, which hid its surface from view until only recently
(Fig. 1). Since Titan is far from the Sun, methane plays
the active role there that water plays on Earth, serving
as a condensable greenhouse gas, forming clouds and
rain, and pooling on the surface as lakes. Titan’s icy surface is shaped not only by impact craters and tectonics
but also by volcanism in which the lava is liquid water
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(“cryovolcanism”), by rivers of liquid methane, and by
tidally driven winds that sculpt drifts of aromatic organics into long linear dunes. Until only 3 years ago, little of
this was known.
The dramatic and surprising results obtained from
the Cassini-Huygens mission2 have shown Titan to be
much more diverse and complicated than had been
thought, making it much more than just an icy satellite
with an atmosphere.
EXPLORATION HISTORY
Titan was discovered in 1655 by the Dutchman
Christiaan Huygens, who realized simultaneously that
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Figure 1. A false-color composite of Cassini International Space
Station (ISS) images. The blue is rendered from a UV filter, making
it particularly responsive to high-altitude haze, which appears
especially abundant over the northern polar region (north is up).
The tan is generated from a near-IR filter in a methane absorption
band: the northern hemisphere has more haze lying above the bulk
of the methane, hence it is more tan. The green channel is from a
near-IR “window” that penetrates to the surface, showing the irregular “coastline” at left. The irregular white patch at the bottom is a
complex of tropospheric clouds around the south pole, which were
observed to move and evolve over periods of a few hours. (Image
courtesy of NASA/JPL/Ciclops/University of Arizona.)
Saturn’s rings (seen only as fuzzy blobs before his highquality telescope was trained on the planet) were just that,
rings defining a plane, and that a satellite, later named
Titan, orbited in that plane once every 16 Earth days.
Huygens was a remarkable thinker and considered that,
absent any information to the contrary, other planetary
bodies must be Earth-like. He even noted that the outer
planets, being so far from the Sun, would be very cold and
thus their clouds and rain would have to be made of something other than water.
Another Dutchman, Gerard Kuiper, working in the
United States in 1944, discovered this other something
on Titan: dark bands in the spectrum of sunlight reflected
by Titan matched those of methane measured in the laboratory. This immediately made Titan unique among the
satellites in the solar system—it had an atmosphere.
Even through the 1970s, with the first space-borne
observations of Titan (in the UV by the Orbiting Astronomical Observatory-2) together with thermal IR observations from the ground, the thickness of this atmosphere
was unknown. The early observations seemed mutually
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contradictory: a thick atmosphere would reflect blue
and UV light more brightly than the space-borne
observations (just as our own sky is blue), but the
warm temperatures implied by the ground-based
thermal data required a thick atmosphere.
Observations from the Voyager 1 spacecraft as it
flew by Titan in 1980 resolved the matter, showing
both “end members” to be partly correct. Voyager
showed Titan’s atmosphere to be like Earth’s, made
predominantly (>90%) of molecular nitrogen and
having a surface pressure of 150 kPa and a temperature around 94 K, indicating its “sea-level” air density
to be 4 times higher than Earth’s.
Methane makes up about 1.5% of Titan’s stratosphere, although (much like water vapor in the
Earth’s troposphere) its abundance increases somewhat nearer the ground. Indeed, the temperature profile indicated by Voyager showed Titan to have a very
Earth-like atmospheric structure, but one that was
colder and vertically stretched in the low gravity, i.e.,
temperatures slowly fall with altitude from the surface
upward to the chilly tropopause at 40 km. And just
as with water vapor on Earth, Titan’s tropopause acts
as a “cold trap” for methane, which is a condensable
greenhouse gas. At higher altitudes, local absorption
of sunlight (on Earth by ozone, on Titan by the haze
discussed below) causes temperatures to rise with altitude, forming a stable stratosphere. It is this warm,
stable stratosphere that causes the large thermal
fluxes observed from Earth.
It had been suspected from Titan’s reddish color
and the polarization of its light that its atmosphere
might be hazy, and indeed it is this haze that prevents
Titan’s sky from being blue. Because it absorbs around
half of the sunlight incident on Titan, the haze is a
major factor in controlling Titan’s climate: together
with the condensable greenhouse gas (methane), the
haze opens Titan to many interesting climate feedback processes.
A further intriguing analogy with Earth is the “polar
hood” observed by Voyager (and also by Cassini; see
Fig. 2). There is a seasonally variable enhancement of
haze opacity over the winter polar regions, which are
dynamically isolated from the rest of the atmosphere
by a circumpolar vortex. A similar effect occurs on
Earth where polar stratospheric clouds form in the
cold winter darkness but are kept from dispersing by
the circumpolar winds, especially around Antarctica.
On Earth, these clouds are instrumental in the catalysis of ozone destruction, and so are responsible for
the formation of the ozone hole. Titan’s chemistry is
very different, but the physical setting and the peculiar processes occurring in its polar night may have
much to teach us about Earth.
Titan’s haze is produced by the destruction of methane by solar UV light and the bombardment of the
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EXPLORATION OF TITAN
Figure 2. Cassini ISS images show striking structure in the atmospheric haze. The north polar region (top) features a particularly
complex structure (the “North Polar Hood”). The main orange
haze deck, as well as the “detached haze” (which appears dynamically connected to the polar hood), has somewhat different optical properties. The lower panel shows a close-up example of the
rich, layered structure in the haze (“up” is to the right.) (Images
courtesy of NASA/JPL/Ciclops.)
atmosphere by energetic particles in Saturn’s magnetosphere. Titan orbits 1.2 million kilometers from Saturn
(20 Saturn radii) so that it is sometimes inside the magnetosphere but outside during periods of stronger solar
wind. Titan’s ionosphere is therefore exposed to various
combinations of sunlight, solar wind, and Saturn’s magnetospheric flow, making it a highly variable and dynamic
region. These energy sources cause heating and drive
winds, and also break methane and nitrogen molecules
apart. These fragments recombine in myriad ways, producing a host of larger organic molecules, some of which
make up the reddish haze.
Post-Voyager models predicted that ethane would be
the dominant product of methane photolytic destruction: the process is irreversible, since hydrogen liberated
from methane escapes into space. The amount of methane presently in Titan’s atmosphere would be depleted
in only a few million years, a fraction of the age of the
solar system. So for Titan’s methane presence today not
to be a quirk, it must be buffered by a surface reservoir.
Both methane and ethane are liquids at Titan’s surface
temperature, and thus an intriguing possibility is that
this reservoir was an ocean of methane, progressively
being converted into ethane over geological time.
In response to these and many other mysteries of
the Saturnian system (the spokes observed in its rings,
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the bright/dark dichotomy on Iapetus, the fresh surface
of Enceladus, and the E-ring, among others) a followon mission was proposed. That mission opened up two
new dimensions compared with Galileo (the follow-on
mission to Jupiter). First, while Galileo’s tour at Jupiter made multiple encounters with the planet’s four
large satellites that orbit in a plane, Cassini’s orbital
tour was three-dimensional—the inclination of the
spacecraft’s orbital plane could be cranked upward to
look down on the rings and Saturn’s polar regions. In
the Saturnian system, only Titan is massive enough
to usefully alter the spacecraft’s orbit (a “slingshot” or
gravity assist), so Cassini’s orbital tour was constructed
by chaining one Titan flyby after another, with each
flyby changing the orbital period, the orientation in
its plane, or its inclination. With a 4-year tour making
many dozens of orbits, the number of possible tours was
enormous.
The second added dimension, that is, international
participation, has been more significant. Although
NASA had studied follow-on Saturnian and Titan missions, notably SO2P (Saturn Orbiter with Two Probes,
one for Titan and one for Saturn), the Cassini mission in
its modern form originated in an international proposal
to the European Space Agency (ESA). Several years of
joint studies followed, culminating in a new start for
Cassini as part (subsequent cancellations were to make
it the only part) of the Mariner Mark II series in NASA’s
program in 1989, and the selection of a Titan probe for
Cassini, to be named Huygens, as ESA’s second mediumclass mission.
In addition to the ESA contribution of the probe and
its support systems, its member states and the United
States provided instruments for both the Huygens probe
and the orbiter spacecraft, making both elements truly
international endeavors. In addition, Cassini’s 4-m-wide
high gain antenna, together with its four-wavelength feed
structure, was supplied by the Italian Space Agency.
By the time Cassini was launched in 1997, the Hubble
Space Telescope (HST) had been refurbished, enabling
it to image Titan in near-IR wavelengths where the haze
was less opaque. These images showed Titan to have a
variegated surface, notably with a large, bright region
named Xanadu on its leading face. This near-IR map—
soon supplemented by others made from larger groundbased telescopes equipped with ever-improving adaptive
optics systems as well as radar observations made with
large radio telescopes—showed that Titan did not have
a global ocean after all. Yet something was making, or
keeping, part of its surface brighter than other parts,
despite an expected slow drizzle of photochemical debris
all over its surface.
HST images showed Titan’s haze to be changing
seasonally: it was thicker in one hemisphere than the other.
In 1980, Voyager showed Titan’s northern hemisphere
to be optically brighter, indicating less haze. In the
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mid-1990s, the HST saw the opposite, as might be
expected half a Titan year later. (Saturn and Titan are
inclined at 26° to the Sun, giving them strong seasonal
forcing, albeit much slower than Earth, since they orbit
the Sun only every 29.6 years.) It seems that air rises
in the summer hemisphere, blowing haze toward the
winter side.
Rising air, much like in the Earth’s tropics, creates
large cumulus clouds on Titan. Indications of cloud
activity were first observed in ground-based spectra and
more tentatively in HST images in 1995. But in the first
years of the new millennium, large ground-based telescopes with adaptive optics showed massive methane
cloud systems around Titan’s south pole. The southern
summer solstice on Titan was in 2003.
Thus our understanding of Titan, and our appreciation of it as an active place, grew as Cassini-Huygens
steadily cruised to Saturn. Even with its formidable Titan
IV launch vehicle, Cassini required flybys of Venus (in
1998 and 1999), Earth (in 1999), and Jupiter (in 2000) to
gain enough energy to reach Saturn in July 2004.
THE EMERGING PICTURE
Before Cassini, expectations of Titan’s surface ranged
from a global ocean of liquid hydrocarbons to a dead,
cratered landscape like Callisto. One leading idea
as to what made Xanadu (a region on Titan comparable in size to Australia) bright was that it might be
mountainous and that methane rainfall might wash
it clean of dark organics. This explanation, however,
was considered speculative. The source of the methane remained a puzzle. Perhaps it was from lakes, seas,
or subsurface “aquifers,” or perhaps it was supplied by
eruption in cryovolcanos.3 In fact, the latter idea does not
allow an easy escape from the notion that there should,
at least sometimes, be surface liquids: unless the methane
supply rate to the atmosphere was exquisitely finely tuned
to just balance the loss by photolysis, the atmosphere
would in time run dry or would periodically become
saturated, allowing lakes and seas to form anyway. Only
observations near Titan would offer the hope of solving
the riddle.
Cassini’s ISS (Imaging Science Subsystem) can observe in the near-IR, like HST, but the haze is still thick
enough at this wavelength (940 nm) to blur images somewhat. The first ISS images showed a complex surface
with bright and dark regions (Fig. 1). In some cases the
boundaries between these regions were irregular, reminiscent of terrestrial coastlines, while some other dark
patches had straight edges.4 An interesting observation
was that the western margins of bright terrain seemed
to be consistently sharp while the eastern margins were
often diffuse, suggesting that bright material might be
transported eastward by winds or other fluid flow.
The discovery of dozens of features on Titan’s
surface required that they be named. Mediated by
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the International Astronomical Union (IAU), the
naming scheme evokes mythology and geography.
Bright regions are named after places of enchantment,
paradise, or celestial realms, and craters are named
after deities of wisdom. Lakes are named after actual
terrestrial lakes, and rivers are named after mythical
or imaginary rivers. Many of these names, such as
Mare Crisium on Earth’s Moon or Meridiani Planum
on Mars, are becoming familiar to planetary scientists.
Figure 3 shows a map made from Cassini ISS images
with major features noted.
With Titan’s haze in mind, Cassini is equipped with
a multimode radar instrument. In addition to large-scale
scatterometry and passive radiometry measurements,
it can perform altimetry and synthetic aperture radar
(SAR) imaging. During close flybys of Titan, SAR allows
Cassini to map long strips covering over a million square
kilometers (about 1% of the surface) on each pass with
resolutions down to 300 m.
The first radar image5 in October 2004 showed an
inscrutable region well north of Xanadu, apparently
containing cryovolcanic and perhaps fluvial features.
Remarkably, no impact features were seen in the above
strips. Since impact features are continuously generated
throughout the solar system, hundreds of craters should
have been seen if the surface was old. Thus the lack
of craters suggested that, like Earth, Titan’s surface is
young, being continuously shaped by processes such as
erosion or volcanism.
HUYGENS
Much anticipation accompanied the descent of the
Huygens probe (Fig. 4). The probe was released on
Christmas Eve, 2004, before making a 21-day dormant
coast initially away from Saturn and then arcing back
inward to enter Titan’s atmosphere at 6 km/s on 14
January 2005. Cassini fired its engine to trail behind the
probe as it flew past Titan to act as a communications
relay during the probe’s descent.
Protected by its heat shield, Huygens decelerated to
Mach 1.5 at an altitude of 160 km, at which point it
deployed its parachutes to make a 2.5-h descent to the
surface. The peak deceleration was only ≈14 (Earth)
g; the large-scale height of Titan’s atmosphere makes
hypersonic entry easier there than anywhere else in the
solar system.
The probe profiled the optical properties of the haze,
finding6 —in contrast to most pre-Huygens models—
that the haze extended all the way down to the surface.
The atmospheric temperature structure, which resembles a cold but vertically stretched version of that of the
Earth, was much as had been predicted from Voyager
radio-occultation data: the surface temperature was
94 K, while a temperature minimum of 71 K at an altitude of 40 km defines the tropopause.
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EXPLORATION OF TITAN
Figure 3. A map of Titan from Cassini ISS images at 940 nm, with names indicated for major regions (names are selected by a committee
of the IAU). Some areas are mapped at rather higher resolution than others; latitudes 50oN and higher are insufficiently illuminated in the
season shown to map. (Image courtesy of NASA/JPL/Ciclops.)
As predicted, the probe drifted some hundreds of kilometers eastward in the zonal winds, although a striking
region of wind shear was discovered at an altitude of ≈80
km, where the wind declined to near zero. Somewhat
unexpected was a reversal in the drift direction in the
Figure 4. (Left) Artist’s impression of the Huygens probe released
from the Cassini orbiter. (Center) The knee-high view of Titan’s
surface acquired by Huygens’ camera after impact. (Right) The
lunar surface shown at the same perspective. (Images courtesy of
ESA/NASA/JPL/University of Arizona.)
Johns Hopkins APL Technical Digest, Volume 27, Number 2 (2006)
lowest few kilometers of descent. Electric field investigations found no definitive evidence of lightning, which
was, in any case, considered unlikely since there is little
sunlight to drive convective activity, and unlike water,
methane is a nonpolar molecule and so is less effective at
charge separation. However, an increase in atmospheric
ionization was observed around a 90-km altitude due to
cosmic rays (whose interaction with nitrogen will also
produce a small amount of radiocarbon, as in the Earth’s
atmosphere).
A much-anticipated moment was the impact of Huygens with the surface—an event for which survival was
never guaranteed. The probe hit the surface at 5 m/s
(about the speed of a laptop dropped from a table on
Earth), encountered a fairly soft surface (mechanically
resembling sand or mud), and continued to operate quite
happily.
Images (taken from knee-height!) of the surface
showed a fluvial landscape, broadly flat with a finegrained dark substrate on which sat rounded, bright cobbles between 5 and 15 cm in diameter. The rounding of
the cobbles and the absence of small pebbles suggested
the area had last been shaped by the rapid flow of liquid,
most probably the result of a methane rainstorm.
The impact embedded the heated inlet of a gas chromatograph/mass spectrometer into the ground. This
fortuitous surface sampling allowed the detection of
methane, ethane, carbon dioxide, and benzene in the
surface material, which doubtless contained a host of
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R. D. LORENZ
other unmeasured organic compounds. An analysis of
the inlet temperature suggested that the ground had to
be physically damp with liquid methane.
An hour after its historic landing, Huygens was continuing to transmit, but the Cassini spacecraft, acting as
Huygen’s data relay, set in the western horizon. Analysis
of the irregular fading of signal strength due to interference of the direct radio signal with a grazing reflection
from the ground showed that the terrain to the west was
similar in roughness to that seen to the south by the
camera. Radio telescopes on Earth were able to detect
continuing transmission for another 2 h: models of battery depletion suggest that probe operations ceased very
soon thereafter.
NOT EVEN THE BEGINNING OF THE END
While clearly a highlight of the Cassini mission, the
Huygens descent merely marked the end of the beginning of a new era of Titan exploration. Cassini is scheduled to make 44 close flybys of Titan during its 4-year
nominal mission, with an additional ≈20 planned for a
2-year initial mission extension.
In terms of revealing details of the landscape (Fig. 5),
radar observations have been instrumental. Radar measurements are made on only about a third of the flybys,
since Cassini lacks a scan platform to independently
point its instruments, which cannot all observe at once.
The massive Cassini spacecraft must therefore be slewed
around as it whips by a target to point its instruments.
The first six radar passes seemed to show a different body
each time! Only a handful of impact craters have been
found, indicating an overall young surface.
River channels running for several hundred kilometers have been observed. Some have the characteristics
of desert washes, formed by rare but heavy downpours.
(Models of methane thunderstorms on Titan show they
can deposit several tens of centimeters of liquid in only a
few hours, much like the heaviest downpours on Earth.)
Other channels are more heavily incised valleys, while
still others are shallow and meandering.
A surprising finding7 was that large, dark areas
near Titan’s equator were not (as was once thought)
liquid, but rather giant sand seas. These sand seas are
filled with massive linear sand dunes, tens of kilometers long, a couple of kilometers apart, and up to 150 m
high—exactly the size and style of linear dunes seen in
the Namib or Arabian deserts on Earth. Linear dunes,
which run along the vector mean wind direction rather
than across it, form in wind regimes that alternate
between two general directions. On Earth this fluctuation is usually seasonal, relating to monsoonal flows, but
on Titan the fluctuation in wind direction is more likely
due to Saturn’s massive gravitational tide in Titan’s
atmosphere. These tidal currents of air exist on Earth,
but are tiny compared with the solar-driven winds on
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our rapidly rotating planet, which heats and cools on a
daily timescale. On slowly rotating Titan, with its massive atmosphere so far from the Sun, the response to
changing sunlight is heavily damped, and the tidal
effects may dominate.
In summer 2006, the first radar images at high northern latitudes revealed dozens of lakes,8 some of which
were simply round while others had the crenellated
appearance of flooded river valleys. The lakes were the
darkest areas in radar images of Titan’s surface and also
showed a high microwave emissivity consistent with a
liquid hydrocarbon composition. Whether the lakes are
concentrated at high latitudes because of lower surface
temperatures there, or because liquid ethane is preferentially deposited near the poles, is not yet clear. Nor is it
known whether the lakes are mostly methane or ethane,
although a large ethane cloud deck has been observed
by Cassini at high northern latitudes, possibly related to
the polar hood, which may supply the lower atmosphere
with condensation nuclei to aid in cloud formation.
Spectral mapping (Fig. 6) by the Visual and Infrared
Mapping Spectrometer (VIMS) has found several distinct
terrain compositions.9 In addition to the bright terrain
making up Xanadu, there seem to be two types of dark
terrain: a “brown” unit characteristic of dune-covered
areas and a “blue” one apparently associated with river
channels. Work is under way to determine the chemical
composition responsible for these different units. A further spectrally distinct surface type is associated with a
few patches on Titan, which are the brightest regions in
a window at a 5-mm wavelength. Spectral identification
of materials from orbit is complicated on Titan because
of scattering by the haze and absorption of much of the
near-IR spectrum by methane gas, the very absorption
bands that permitted the discovery of Titan’s atmosphere
in the first place. Relatively few materials are bright
at this wavelength; water ice, for example, is not. One
intriguing possibility is carbon dioxide ice, which might
be associated with cryovolcanism. Active jets have been
observed spewing water vapor and traces of other gases,
including carbon dioxide, on Enceladus, one of Saturn’s
smallest moons.
CASSINI CONTINUES
At the time of writing, Cassini is halfway through
its 44 nominal mission flybys, and its orbital plane is
being flipped over to change the viewing geometry of
the many scientific targets at Titan. Toward the end of
the nominal mission in July 2008, the inclination of the
orbit will be progressively increased, allowing Cassini to
look down on the rings and onto Saturn’s high latitudes
where aurora occur.
Overall, the spacecraft—already in space for a
decade—is in good health and has ample propellant
to support further operations. As noted earlier, a 2-year
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EXPLORATION OF TITAN
Figure 5. A montage of Cassini radar images showing the diversity of landforms on Titan’s surface at a common scale (most locations are
indicated in Fig. 3). North is roughly upward in all cases: (a) Lakes near 80oN latitude, (b) the 440-km impact structure Menrva and network
of braided river channels to the east, (c) giant linear sand dunes in the near equatorial region Belet, (d) a dendritic river channel network
in the western end of Xanadu, (e) the 80-km impact crater Sinlap, (f ) Ganesa Macula, a possible cryovolcanic dome, with bright fans to the
west, (g) bright mountain chains in Adiri forming a chevron pattern, (h) a lobate structure, possibly a flow, seen on the first close flyby, and
(i) dunes sweeping around lobate topography in northwestern Belet. (Images courtesy of the author and the Cassini Radar Team.)
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R. D. LORENZ
Figure 6. A mosaic of VIMS data showing the spectral diversity of Titan’s surface. The bright orange areas, notably Tui Regio and
inside Hotei Arcus, are particularly reflective at 5 mm, perhaps indicating CO2-rich deposits that might be associated with cryovolcanism. In this mosaic, bright clouds are present around the south pole. (Image courtesy of NASA/JPL/University of Arizona.)
extended mission is presently being planned, permitting
over 20 more Titan flybys as well as much other science
in the Saturnian system as a whole.
In addition to building up mapping coverage at all
wavelengths, the Cassini encounters aim to determine
Titan’s internal structure from its gravity field via precise
radio tracking and perhaps to discern hints of an internal water ocean. Beyond observations of cryovolcanic
features, clues to such an ocean might include a change
in shape and gravity field through flexing of the crust
through one tidal cycle as well as measurements of any
induced magnetic field. Those measurements (which
were used by Galileo to indicate water oceans on Europa
and Ganymede) are more challenging at Titan because
of weaker changes in the Saturn field there (the inductive detection of a conducting interior, by implication
liquid water, requires modulation of the applied field)
and because the ionosphere above Titan’s thick atmosphere blocks the internal signature.
While Cassini’s flybys have predominantly imaged
Titan’s northern hemisphere, the extended mission will
allow much more of the south to be mapped. An exciting prospect is to observe Titan around its northern
spring equinox in 2009: the atmospheric circulation may
undergo a convulsive change from the gentle Hadley
circulation that transports haze from the summer to
winter hemisphere. Models suggest that for a year or
two a double-cell equator-to-pole symmetric circulation
may develop before being replaced by the new (reversed)
pole-to-pole circulation.
One possibility being considered is to introduce particularly low-altitude encounters to get beneath the ionosphere and sample the atmospheric chemistry at higher
densities than has been dared in the nominal mission.
This chemistry (Fig. 7, Table 1) has been found to be
much more complex than anticipated, showing an abundance of compounds with masses of 70 and above such
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as benzene.11 Although some 20 or so different molecules
had been detected on Titan (principally with mid- and
far-IR spectroscopy by Voyager and from Earth) before
Cassini arrived, it had been assumed that only simple
molecules would be present at the ≈1000-km altitudes
where Cassini flies past.
Clearly, Cassini’s findings have found our understanding of this chemistry to be wanting. The richness of the
upper atmospheric chemistry may be connected with
the surprising balance of surface materials. A relatively
modest coverage by surface liquids has been observed
so far, although some lakes (Fig. 8) are several hundred
kilometers across and thus may qualify as “seas.” In
Figure 7. A mass spectrum acquired by Cassini’s Ion and Neutral Mass Spectrometer between an altitude of 1100–1300 km as
it flew through the upper atmosphere. The spectrum shows the
surprising abundance of large hydrocarbon molecules, even at
altitudes above 1000 km. This material forms Titan’s haze and, perhaps growing in complexity as it descends, is deposited on Titan’s
surface. (Data courtesy of NASA/SwRI.)
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EXPLORATION OF TITAN
Table 1. Composition of Titan’s atmosphere (after Ref. 10).
Common (official) name
Formula
Amount (in stratosphere unless
otherwise indicated)
Nitrogen
N2
Methane
CH4
Hydrogen
H2
Argon
Ar40
Ar36
Ethane
C2H6
Carbon monoxide
CO
Acetylene (ethyne)
C2H2
Propane
C3H8
Hydrogen cyanide
HCN
Ethylene (ethene)
C2H4
Carbon dioxide
CO2
Methyl acetylene (propyne)
C3H4
Acetonitrile
CH3CN
Cyanoacetylene
HC3N
Methyl acetylene
CH3C2H
Cyanogen
C2N2
Water vapor
H2O
Diacetylene (buta-1,3-diyne)
C4H2
Benzene
C6H6
95% near surface
≈98%
4.9% at surface near equator
1.4% in lower stratosphere
≈2% at ≈1000 km
0.1–0.2% in lower atmosphere
≈0.4% at ≈1000 km
43 parts per million (ppm)
28 parts per billion (ppb)
≈20 ppm
≈45 ppm
3.3 ppm
19 ppm at ≈1000 km
700 ppb
800 ppb in winter stratosphere
≈100 ppb in summer stratosphere
160 ppb
15 ppb
10 ppb
a few ppb
>5 ppb in winter stratosphere
<1 ppb in summer stratosphere
5 ppb
5 ppb
8 ppb
1.5 ppb (slightly higher in winter)
1.4 ppb at winter pole, <0.5 ppb elsewhere
Figure 8. This radar image shows a lake/
coastline region at around 79oN, 310oW.
An island (90 3 150 km) is mountainous (as evidenced by the “layover” of the
peaks toward the top of the image, from
which direction the radar observation was
made). It is bounded at the top by a pitchblack area that is probably liquid and on
the lower side by a lighter-shaded channeled area, perhaps a tidal mudflat.
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R. D. LORENZ
contrast, vast sand seas of dark dunes cover some 40% of
low-latitude terrain, perhaps a fifth of the entire surface
area of Titan.
At the end of a 2-year mission extension, Cassini
could easily be introduced into a low-maintenance
cycler orbit between Titan and Enceladus. With minimal intervention, it would make repeated flybys of
both bodies, yielding an efficient science return as
long as its systems, propellant, and ground support
hold out.
OUTSTANDING QUESTIONS AND FUTURE MISSIONS
better resolution than Cassini and with much more
complete coverage. A global topographical dataset, to
which Cassini will contribute little, could be generated
by an orbiting altimeter and would enable a quantitative
study of Titan’s meteorology and hydrology, measuring
the slopes that drive winds and river flow, and perhaps
even detecting the ebb and flow of tides in the polar
lakes. Prolonged immersion in the near-Titan environment would allow a full exploration of the variations
of composition and structure in the upper atmosphere
with latitude, altitude, and orbital position and permit
the unraveling of the various contributions to the atmospheric “chemical factory” from magnetospheric electrons, the solar wind, and UV light.
An important enhancing, if not enabling, technology
for an orbiter is aerocapture. To brake into orbit around
Titan requires a substantial velocity change (delta V) of
some 3–4 km/s, which would entail a formidable propulsion system but can be readily achieved by dipping into
the atmosphere upon arrival. This is a natural extension
of the aerobraking procedure that is now routinely used at
Mars. The large-scale height at Titan makes the navigational accuracy for safe aerocapture easily achievable and
keeps deceleration and heat loads down to modest values.
Despite the formidable science return possible from
an orbiter, it is in situ exploration that is likely to capture the imagination at Titan, much as the Huygens
probe did. A lander (Fig. 9) would be a natural way to
measure the composition of the surface material, which
the Huygens probe showed to be rich in organic matter.
Huygens’ instrumentation was not designed for detailed
surface measurements, yet it is on the surface that the
Cassini is a mission to the Saturnian system, not to
Titan. Even with the outstanding return from Cassini’s
formidable array of instruments on its Titan flybys, Cassini spends only an hour within 10,000 km of Titan on
each flyby. It is likely that radar imaging after the 2-year
extended mission will only reach 30–40% coverage, and
high-resolution spectral mapping will have sampled only
small areas. Much is left to discover at Titan, and the
long gestation and delivery time of missions to the outer
solar system is such that it is already time to consider
what future missions might accomplish.
There are many isolated scientific questions to consider, but above all is the notion of how Titan functions
as a system. A nested cycle fluxes carbon atoms around.
In an inner hydrological cycle, methane forms clouds and
rain, carving channels on the surface and perhaps accumulating in polar lakes before evaporating again. Over
longer periods, the methane must come from Titan’s
interior, released perhaps from clathrate ice reservoirs
through vents or cryovolcanos. In the upper atmosphere,
the methane and nitrogen are
destroyed and converted into
heavier organic molecules
which ultimately are deposited
on the surface. Identifying the
sources, sinks, and pathways in
this complex system requires
Titan to be studied at a range of
scales from top to bottom, from
its ionosphere to its interior.
Orbiting Titan, rather than
flying past, would allow a future
mission to get more than fleeting glances at the body. Indeed,
within the first 3 days of a
dedicated follow-on mission to
Titan, the mission would spend
Figure 9. Artist’s impression of a lander sitting amid Titan’s dune fields. The sampling arm
longer at Titan than Cassini.
is equipped with a UV lamp to excite fluorescence in specific organic compounds. A seisAn orbiter with modest capabilmometer/magnetometer package lies in the foreground, connected to the lander by a tether,
ities could, over only a couple of
and a meteorology mast is at left. (Artwork by James Garry [www.fastlight.co.uk]; used with
years, conduct radar and nearpermission.)
IR mapping with 3–10 times
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EXPLORATION OF TITAN
most complex astrobiological materials are likely—perhaps where cryovolcanic materials have interacted with
the organic haze. Measurements might include gas or
liquid chromatography and mass spectrometry to look
for prebiotic molecules such as pyrimidines (found in
DNA) and amino acids (found in proteins), which
laboratory experiments show might be made on Titan.
Another important task for the onboard chemical laboratory is to assess how enantiomeric the surface organics
are, i.e., whether left- and right-handed stereochemicals
are produced in equal proportion or (as in living matter
on Earth) one stereoisomer predominates. Another goal
might be to measure the radioactivity of the material,
since cosmic ray interactions with nitrogen in the lower
stratosphere will produce (as on Earth) radiocarbon. It
may therefore be possible to determine how “fresh” the
haze material is.
A simple seismometer could probe Titan’s internal
structure and would probably find a substantial excitation of crustal flexing by the changing tidal potential
over a 16-day orbit of Titan around Saturn. A small
meteorological package could also monitor changes in
the winds, which likely also have a substantial tidal
component.
Titan’s surface is diverse, and no practical number
of landers can catalog all of its different surface types.
Large-scale mobility is the obvious solution, and as experience with Mars rovers has shown, engages the public in
ways that remote sensing does not. Titan’s environment
allows one to consider many possible vehicle types,12
most notably aerial platforms, that permit mobility
over long distances. Suggestions of balloons at Titan
date back to 1978, and serious studies have been made
since 1983.
A leading idea to tackle this diverse surface is a
montgolfière (a hot air balloon). Buoyancy via warmth
is a particularly efficient strategy on Titan with its cold,
dense atmosphere, and is easily achieved. A radioisotope
power source, which is in any case necessary for a longduration mission so far from the Sun, tends to have a
modest conversion efficiency of 5% or so. Thus to produce 100 W of electricity, around 2 kW of heat must be
rejected from the source, but this heat can be exploited
to warm the balloon gas. By modulating the heat flow,
the altitude of the balloon could be controlled, much
as is done with ballast and propane burners on terrestrial hot air balloons. And as knowledge of Titan’s wind
improves, it may be possible (as on Earth) to exploit different wind speeds and directions at different altitudes
to develop some control over where the balloon drifts
(Fig. 10).
Over a likely lifetime of a year floating at an altitude
of a few kilometers, the balloon would circumnavigate
Titan two or more times. Key science on a balloon would
be imaging of the surface and lower atmosphere. The
“airplane window” perspective afforded by the balloon
Johns Hopkins APL Technical Digest, Volume 27, Number 2 (2006)
Figure 10. (Top) Artist’s impression of a hot-air balloon at Titan.
(Bottom) A simulation of the trajectory of a balloon floating at a
fixed 8-km altitude simulated using a general circulation model of
Titan’s winds. The plot shows the trajectory over several months
overlaid on a low-resolution map of Titan.
would give a high-resolution stereo view of hundreds or
thousands of locations. High resolution is not as easy to
achieve from an orbiter high above Titan’s hazy atmosphere, as has been done at Mars, and imaging from a
balloon fills an important scale gap between orbiter and
lander observations. A subsurface radar sounder could
measure topography and detect subsurface layers, such
as the depth of the sand in the sand seas or the depth
of lakes, and perhaps detect any subsurface liquid water
ocean. It would, furthermore, be a tremendously exciting
platform for education and outreach. Plotting updates on
its location on a wall-chart map of Titan could become
a daily activity in schools, together with launching
balloons.
In fact, a Flagship-class mission might include all of
these elements. There are significant technical advantages to such a combination: a lander acting as a fixed
point can provide an important navigational reference, and an orbiter can dramatically improve the data
volume returned from a balloon and/or lander by acting
as a relay. The scientific synergies are no less important;
for example, the drifting of the balloon and the changing winds at a fixed lander location provide powerful
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R. D. LORENZ
independent constraints on the meteorology, while
the lander/orbiter combination gives a top-to-bottom
picture of the production and deposition of organic
material. Such a multipronged approach gives a substantial robustness to what must be a long-duration mission, since each element by itself can provide a significant return. Also, the opportunity for cost-sharing with
international partners exists, and through Cassini there
is already an established community of European scientists with interests in Titan.
CONCLUSIONS
In addition to the ice geology and space physics
that Titan shares with other outer planet satellites, it
also shares many other fields. Hydrologists and Aeolian sedimentologists can engage Titan as well as Earth
and Mars, and limnology and oceanography are now
no longer just Earth sciences. Titan’s weather—with its
methane downpours in a greenhouse climate and peculiar polar photochemistry—may offer some significant
and relevant insights into processes that affect us here
on Earth. Titan is also the chemically richest place in
the solar system, and thus promises to give important
insights into the generation of chemical complexity in
the universe, a set of processes that ultimately led to the
formation of life.
Titan kept many of its secrets shrouded in haze
through the first decades of space exploration but is now
beginning to reveal itself as one of the most diverse,
complex, and Earth-like places in the solar system.
NASA has recently initiated a study at APL to examine
what a Flagship-class mission dedicated to Titan could
accomplish. This bizarre orange-colored world has a rosy
future.
REFERENCES
1Lorenz,
R. D., and Mitton, J., Lifting Titan’s Veil, Cambridge University Press (2002).
2Harland, D. M., Mission to Saturn, Praxis-Springer (2003).
3Sotin, C., Jaumann, R., Buratti, B. J., Brown, R. H., Clark, R. N., et al.,
“Release of Volatiles from a Possible Cryovolcano from Near-Infrared
Imaging of Titan,” Nature 435, 786–789 (2005).
4Porco, C. C., Baker, E., Barbara, J., Beurle, K., Brahic, A., et al.,
“Imaging of Titan from the Cassini Spacecraft,” Nature 434, 159–168
(2005).
5Elachi, C., Wall, S., Allison, M., Anderson, Y., Boehmer, R., et al.,
“Cassini Radar Views of the Surface of Titan,” Science 308, 970–974
(2005).
6Lebreton, J.-P., Witasse, O., Sollazzo, C., Bancquaert, T., Couzin, P.,
et al., “Huygens Descent and Landing on Titan: Mission Overview
and Science Highlights,” Nature 438, 758–764 (2005). (Key scientific
results from the experiments onboard Huygens are given in six papers
in the subsequent pages of that journal.)
7Lorenz, R. D., Wall, S., Radebaugh, J., Boubin, G., Reffet, E., et al.,
“The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes,” Science 312, 724–727 (2006).
8Stofan, E. F., Elachi, C., Lunine, J. I., Lorenz, R. D., Stiles, B., et al.,
“The Lakes of Titan,” Nature 445, 61–64 (2007).
9Barnes, J. W., Brown, R. H., Soderblom, L., Buratti, B. J., Sotin, C.,
et al., “Global-Scale Surface Spectral Variations on Titan Seen from
Cassini/VIMS,” Icarus 186(1), 242–258 (Jan 2006).
10Waite, J. H. Jr., Niemann, H., Yelle, R. V., Kaspezak, W. T., Cravens,
T. E., et al., “Ion Neutral Mass Spectrometer Results from the First
Flyby of Titan,” Science 308, 982–986 (2005).
11Lorenz, R. D., “Post-Cassini Exploration of Titan: Science Rationale and Mission Concepts,” J. Brit. Interplanet. Soc. 53, 218–234
(2000).
12Lorenz, R. D., and Mitton, I., Titan Unveiled, Princeton University
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The Author
Ralph D. Lorenz received a B.Eng. in aerospace systems engineering from the University of Southampton, UK, in 1990,
when he joined the European Space Agency in the Netherlands to work on Phase B of the Huygens project. He subsequently
obtained a Ph.D. in physics from the University of Kent at Canterbury, UK, where he built part of Huygens’ Surface Science Package experiment. Dr. Lorenz then joined the Lunar and Planetary Laboratory
at the University of Arizona where, in addition to working on Huygens, he joined the Cassini Radar
team, worked on Hubble Space Telescope observations of Titan, and was involved in the Mars Polar
Lander and Deep Space 2 penetrators. After 12 years in Arizona, he joined APL in August 2006. Dr.
Lorenz is the author of several books including Lifting Titan’s Veil, Space Systems Failures, and Spinning
Flight. His latest book, Titan Unveiled, will be published by Princeton University Press later this year. His
e-mail address is ralph.lorenz@jhuapl.edu.
Ralph D. Lorenz
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